Magnesium

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Other names/abbreviations:

Mg

Element no:	12
Element group:	2 (IUPAC) II A (American labelling) II A (European labelling) Alkaline earth metals
Element type:	Metals
CAS-Number:	7439-95-4
EINECS-Number:	231-104-6

Chemical properties

Reaction of magnesium with acids
Magnesium metal dissolves readily in acids forming Mg(II) ions and hydrogen, H₂.

Mg (s) + H₂SO₄ (aq) Mg²⁺ (aq) + SO₄^2- (aq) + H₂ (g)
Mg (s) + 2 HCl (aq) Mg²⁺ (aq) + 2 Cl^- (aq) + H₂ (g)


Reaction of magnesium with air
Magnesium react with oxygen at room temperature, forming a passivating layer of MgO on the surface. When ingited, Mg reacts with both oxygen and nitrogen forming a mixture of magnesium oxide, MgO, and magnesium nitride, Mg₃N₂.

2 Mg (s) + O₂ (g) 2 MgO (s)
3 Mg (s) + N₂ (g) Mg₃N₂ (s)


Reaction of magnesium with bases
Mg(II) is precipitated by hydroxide ions. Ammonia will also precipitate Mg(II) as hydroxide, but the precipitation is incomplete

Mg²⁺ (aq) + 2 OH^- (aq) Mg(OH)₂ (s)


Reaction of magnesium with carbonates
Mg(II) is precipitated by carbonate ions as an alkaline carbonate.

Mg²⁺ (aq) + CO₃^2- (aq) MgCO₃ (s)


Reaction of magnesium with phosphates
Mg(II) is not precipitated by phosphate ions under acetic acid conditions. Under neutral conditions it is precipitated as the hydrogen phosphate

Mg²⁺ (aq) + HPO₄^2- (aq) MgHPO₄ · 7 H₂O (s)

In the presence of ammonia, an ammonium phosphate is precipitated

Mg²⁺ (aq) + NH₃ (aq) + HPO₄^2- (aq) MgNH₄PO₄ (s)


Reaction of magnesium with sulfides
Mg(II) is not precipitated by hydrogen sulfide. A precipitate can sometimes be observed due to the alkaline properties of hydrogen sulfide

S^2- (aq) + H₂O (l) HS^- (aq) + OH^- (aq)
Mg²⁺ (aq) + 2 OH^- (aq) Mg(OH)₂ (s)


Solubility
Water: Insoluble, slow decompositions as powder [4]

Quantitative analysis

Method 3500-Mg C Inductively Coupled Plasma Method [1]. A portion of the sample is digested in a combination of acids. The digest is aspirated into an 8,000 K argon plasma where resulting light emission is quantified for 30 elements simultaneously.

Method limit of detection in water = 0.01 mg/L
Method limit of detection in soil = 100.0 mg/kg

Safety

Foil
Symbol: -
R-phrases: -
S-phrases: -

Powder
Symbol: F
R-phrases: 15-17
S-phrases: 7/8-43

Physical properties

Appearance
	Physical state @ 20°C:	Solid [4]
	Color:	Silvery [4]
	Odor:	Odourless [4]

Bulk properties
	Molecular weight (g/mol):	24.3050(6) [2]
	Melting point (°C):	650 [2]
	Boiling point (°C):	1090 [2]
	Ignition temperature (°C):	> 500 (powder) [1]
	Density (g/cm3):	1.738 [2]
	Molar volume (cm3):	14.00 (20 °C) [3]
	Elastic properties: Young's modulus (GPa): Rigidity modulus (GPa): Bulk modulus (GPa): Poisson ratio:	45.1-45.7 [3] 17.3 [3] 35.6 [3] 0.29-0.33 [3]
	Hardnesses: Mineral hardness:	2.5 [3]

Thermodynamic properties
	ΔfH°gas (kJ/mol):	146 (Monoatomic) [2] 147.1 [5]
	ΔfH°solid (kJ/mol):	0.0 [5]
	ΔH°fusion (kJ/mol):	8.48 [5]
	ΔH°vaporization (kJ/mol):	127.4 [2]
	S°gas (J/K·mol):	148.6 [5]
	S°solid (J/K·mol):	32.7 [5]
	ΔfG°gas (kJ/mol):	112.5 [5]
	Cp (gas) (J/K·mol):	20.8 (25 °C) [5]
	Cp (solid) (J/K·mol):	24.720 (20 °C) [3] 24.869 (25 °C) [5] 25.737 (100 °C) [3] 26.856 (200 °C) [3] 28.992 (400 °C) [3]
	Coeff. of linear thermal expansion (106 K-1):	26.0 (20 °C) [3] 24.8 (25 °C) [5] 26.1 (100 °C) [3] 27.0 (200 °C) [3] 28.9 (400 °C) [3]

Crystal structure
	Crystal type:	HCP [3]

Electronic properties
	Electron configuration:	1s2-2s2-2p6-3s2

Conductivity
	e°(V):	-2.356 [2]
	Electrical resisitvity (μΩ·cm):	4.20 (20 °C) [3] 4.51 (25 °C) [3] 5.60 (100 °C) [3] 7.20 (200 °C) [3] 12.10 (400 °C) [3]
	Thermal (W/m·K):	167 (20 °C) [3] 156 (27 °C) [5] 167 (100 °C) [3] 163 (200 °C) [3] 130 (400 °C) [3]

References

1: Standard Methods for the Analysis of Water and Wastewater, APHA, 1992, 18th edition

2: Greenwood,N.N., Earnshaw,A. Chemistry of the elements
2nd edition (1997) Edited by Greenwood,N.N., Earnshaw,A. pp. 1-1340, Butterworth-Heinemann. Oxford. Great Britain

3: Buch,A. Pure Metals Properties. A Scientific-Technical Handbook
1st edition (1999) Edited by Buch,A. pp. 1-306, ASM International and Freund Publishing House Ltd. Ohio. USA

4: Merck. ChemDAT The Merck Chemical Database Ver. 1.1.5

5: CRC Handbook of Chemistry and Physics
75th edition (1994) Edited by Lide,D.R. , CRC Press Inc. Boca Raton, USA

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© Michael Pilgaard
Created: October 3, 2008
Last update: October 10, 2008